One of the many as yet unresolved problems which remain with the newly developed high T.sub.c superconducting compositions, for example of the 1-2-3-O.sub.7 and 2-2-2-3-O.sub.10 types, is the extremely anisotropic nature of electrical current flow therethrough. While these superconducting compositions have been said to be revolutionary because of the high currents densities which they can carry at above liquid nitrogen temperatures, it has now been proven that the current preferentially flows along the Cu-O plane. This anisotropic current flow has been one of the major impediments to the development of practical electrical devices employing those superconducting compositions. It is currently foreseen that the first applications of the new high T.sub.c superconductors will not be for power storage, but rather to form very sensitive, low loss, low impedance waveguides for transmitting and receiving microwave signals for outer space applications where weight reduction vis-a-vis, copper waveguides is significant. See an article in Supercurrents, July, 1989, pp. 66-71 by Bob Hammond entitled "Passive Microwave Devices and High-T.sub.c Superconductors." In this application, the interior walls of the waveguide will be formed of superconducting compositions deposited upon low loss dielectric substrates. The low loss nature of the substrate is essential. This is because the electric field generated by the microwaves penetrates some finite distance into the substrate. Therefore, even if the superconducting composition is anisotropic and can therefore carry current with minimal complex impedance loss, the lossy substrate, itself, attenuates the microwave energy and represents a source of loss if it is not transparent to those microwaves. Accordingly, one should appreciate that in order to develop low loss superconducting structures, two features are required: (1) a high current-carrying capacity superconducting composition which is aligned in at least the current carrying direction, and (2) a substrate fabricated from a low loss dielectric material upon which the superconducting composition can be deposited. The superconducting structures of the instant invention fulfill both of these requirements.
Until the instant invention, the high current carrying capability of the high T.sub.c, 1-2-3-O.sub.7 type of superconducting composition has only been demonstrated either in (1) relatively small single crystals; or (2) with thin films epitaxially grown on relatively high dielectric loss substrates, such as SrTiO.sub.3, KTaO.sub.3, etc. The methods and superconducting structures of the instant invention overcome these problems, allowing for the deposition of a high quality, C-axis oriented, preferably polycrystalline, Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 (F)-type superconducting composition on low loss dielectric sapphire substrates. In this regard, it is noted that in commonly assigned U.S. patent application Ser. No. 043,279, filed Apr. 27, 1987 of Ovshinsky, et al now abandoned, the assignee of the instant invention first disclosed the use of a fluorinated precursor powder (such as BaF.sub.2) to both stabilize and orient the grains of 1-2-3-O.sub.7 perovskite ceramic oxide type superconducting compositions for improving the current carrying capacity thereof along a preferred direction. However, those powder metallurgically prepared thick films and pellets were multi-phased. It was not until the use of fluorine in the target employed in the laser ablative technique of the instant invention, detailed hereinafter, that there was disclosed the growth of single phase, oriented superconducting films on low loss dielectric substrates.
In this regard, it is important to note the instant inventors do not claim to have discovered the deposition of 1-2-3-O.sub.7 type of superconducting compositions on a Al.sub.2 O.sub.3 substrate. While other researchers have deposited 1-2-3-O.sub.7 compositions on a sapphire substrate, those other researchers have been unable to grow large grains by any process other than an epitaxial one. However, in order to grow large oriented grains in an inexpensive manner, it is essential to separate the 1-2-3-O.sub.7 composition from the aluminum and oxygen elemental components of the sapphire substrate. If these components are not separated, the barium from the unit cells of the superconducting composition chemically reacts with the substrate, thereby (1) destroying the unit cells for a large thickness of superconducting composition growth and (2) creating a plurality of non-superconducting phase compositions which interferes with the growth of the superconducting phase composition. When other researchers have attempted to interpose a barrier layer between the superconducting composition and the substrate to prevent the interdiffusion of elements, the structure of the barrier layer did not readily lend itself to the growth of the superconducting 1-2-3-O.sub.7 phase thereupon.
These and other improvements and advantages of the instant invention will be more fully understood after carefully perusing the detailed description which follows.